1. Field of the Invention
The present invention relates to an electrical double layer capacitor, and more particularly to a large-capacitance electrical double layer capacitor using a solid polarized electrode.
2. Description of the Prior Art
An electrical double layer capacitor in a conventional example 1 will be described with reference to FIGS. 1 to 3.
Referring to FIG. 1, a cell laminate 171 of the electrical double layer capacitor has the structure of laminating two cells. Each cell 161 comprises a gasket 151 of a frame shape, a couple of polarized electrodes 110 deposited within the gasket 151, a separator sandwiched between the polarized electrodes 110 and a current collectors 121 adhered to the upper surface of the gasket 151. An electrolytic solution 130 is sealed in the cell 161. The polarized electrodes 110 contact with the current collectors 121. The upper and the lower surfaces of the gasket 151 are flat as shown in FIG. 2.
The polarized electrodes 110 are made of solid activated carbon mainly containing activated carbon/polyacene material or the like as disclosed in Japanese Patent Application Laid-open No. Hei 4-288361. The current collector 121 is made of rubber or plastic, which contains electrically conductive carbon.
Because a withstand voltage of the electrical double layer capacitor is limited by the electrolysis voltage of the electrolytic solution 130, cells 161 are connected in series in accordance with a required withstand voltage. Also, as shown in FIG. 3, both sides of the cell laminate 171 are sandwiched between pressure plates 190 to apply a pressure between the cells and between terminal electrodes 180, thereby lessening a contact resistance between the current collector 121 and the polarized electrode 110.
In recent years, the electrical double layer capacitor thus structured is made large in capacitance by using an improved polarized electrode, and new usage of the electrical double layer capacitor is found by lessening an equivalent series resistance (hereinafter referred to as xe2x80x9cESRxe2x80x9d). As its examples, there are a usage of a power supply for driving a starter motor in an automobile in combination with a lead-acid battery, and an intended use of an auxiliary power supply in combination with a solar battery.
In the case where the electrical double layer capacitor is used for those intended uses, there is a high possibility that the electrical double layer capacitor is located under a high temperature environment, and the high reliability of the electrical double layer capacitor is required under such an environment. However, the above-described electrical double layer capacitor in the conventional example 1 has the following problems.
(1) In the cell laminate 171, the electrolytic solution 130 within the cell 161 is sealed with the adhesion of the gaskets 151 and the current collectors 121 which are different in material from each other. At a high temperature, the electrolytic solution 130 is thermally expanded. Also, a gas is produced within the cell by the application of a large voltage or a high temperature. As shown in FIG. 1, the outer peripheral end surfaces of the current collectors 121 are exposed to the side surfaces of the cell laminate 171. Because of this structure, there occurs a gap between the gaskets 151 and the current collectors 121 due to their separation caused by the thermal expansion of the electrolytic solution and the occurrence of a gas within the cell, thereby being liable to leak the electrolytic solution 130 within the cell to the exterior from that gap.
(2) In order to improve the electric contact of the polarized electrodes 110 and the current collectors 121, the cell laminate 171 is so structured as to be pressurized from the outside at the outermost current collector 121 and nipped by high rigid metal plates which are hardly deformable (refer to the pressure plates 190 in FIG. 3). Also, among the structural materials of the electrical double layer capacitor, there are many cases in which the polarized electrodes 110 are formed of hard and rigid members such as a sintered body of the activated carbon, and the gaskets 151 are made of a hard material such as ABS resin in order to enhance a precision in the dimensions of a product. Because the current collectors 121 are made of an electrically conductive rubber, which is thin and elastic, a positional displacement occurs when pressurizing the current collectors 121. As a result, a strong force is locally exerted on the current collectors 121, and a crack is liable to occur in the current collectors 121.
A technique by which the above problems are solved is disclosed in Japanese Patent Application laid-open No. Hei 8-78291 (a conventional example 2), Japanese Utility Model laid-open No. Sho 61-117238 (a conventional example 3) and Japanese Patent Application Laid-open No. Hei 5-46026 (a conventional example 4).
In the electrical double layer capacitor of the conventional example 2, as shown in FIG. 4, the outer diameter of a current collector 122 within a cell 162 of a cell laminate 172 is smaller than the outer diameter of the current collector 121 at the outermost side. That is, the electrical double layer capacitor is structured such that the outer diameter of the current collector 122 within the cell is set to have an intermediate value between the inner diameter and the outer diameter of a gasket 152, and the peripheral edge portion of the current collector 122 is received in a recess 153 of the inner peripheral surface of the gasket 152, as shown in FIG. 5.
In the electrical double layer capacitor of the conventional example 3, as shown in FIG. 6, step portions are formed on inner peripheral portions of both end surfaces of gasket 154, as shown in FIG. 6, a current collector 123 is received in these step portions 58, and a cell is composed in the electric double layer capacitor. Two or more cells are stacked and a cell laminate is formed in the conventional example 3.
Also, in the electrical double layer capacitor of the conventional example 4, as shown in FIG. 7, a concave is formed over each of the frame entire periphery on both of the upper and lower surfaces of gaskets 155 of a cell 163 of a cell laminate 173. A projection is formed on a current collector 124 corresponding to the concave, and the projection engages with the concave. FIG. 8 is a schematic view of the gasket 155. A concave 156 is formed over the frame entire periphery on both of the upper and lower surfaces of gasket 155.
In the electrical double layer capacitor of the conventional example 2 to the conventional example 4, there prevents the leakage of the electrolytic solution due to a crack or separation by enhancing the seal strength between the respective cells.
However, the capacitors of the conventional example 2 to the conventional example 4 suffer from the following problem in that it is difficult to improve the productivity and to ensure the reliability.
After the side edge portion of collector 122 in the cell is inserted in the concave portion of gasket 152, gasket 152 is compressed in manufacturing the electric double layer capacitor of the conventional example 2. As a result, the bonding strength of collector 122 and gasket 152 is improved, and the sealing reliability in the electric double layer capacitor of the conventional example 2 is improved, compared with that in the above-mentioned capacitor of the conventional example 1.
However, both the upper and lower end surfaces of the gasket 152 are flat as shown in FIG. 5 and a portion of concave portion 153 of the gasket 152 in which the side edge portion of the current collector 122 is received is thick because the thickness of the current collector 122 exists, and a portion of the concave portion 153 into which the current collector 122 is not inserted is thin. Since a distortion occurs on a boundary with different thickness, the current collector 122 and the gasket 152 are liable to be separated from each other and leak solution, thereby not being capable of ensuring reliability.
Also, in the capacitor of the conventional example 2, the current collector 121 at the outermost side has a structure of sealing the gas with the gasket 152 and the current collector 121 which are different in material from each other, and the sealing structure is identical with that of the capacitor in the conventional example 1 of FIG. 1, and therefore, a pressure load is liable to be more applied to the current collector 121 toward the outermost side under a high temperature environment, as a result t that the solution is liable to be leaked due to a separation between the gasket 152 and the current collector 121. In addition, there is the high possibility that a positional displacement may occur in assembling the current collectors 122 within the cell. The positional displacement leads to the non-uniform pressure resulting in cases where ESR of the cell laminate rises and the solution is leaked from the cell laminate. Thus, there arises such a problem as the decrease of the yield of the capacitor, that is, the decrease in productivity.
In the electrical double layer capacitor of the conventional example 3, the current collectors 123 are received in the step portions 58 formed on inner peripheral portion of both end surface of gasket to result in the improvement of bonding strength of the current collector to the gasket, compared with that in the conventional example 2. However, the current collector the outside of the cell laminate might flake off from the gasket and the electrolytic solution 130 leak outside the capacitor under the environment of the high temperature. Moreover, there as a problem in the capacitor of the conventional example 3 that the electric contact reliabilities between cells of the cell laminate decreased under the high temperature so far.
Then, let us consider the electrical double layer capacitor of the conventional example 4. In this capacitor, the insertion of the projection of the current collector 124 into the concave 156 of the gasket 155 makes it difficult to produce the positional displacement of the current collector 124 when a pressure is applied from the current collector of the outermost layer of the cell laminate. However, when the thickness of the projection of the current collector 124 becomes larger than the depth of the concave of the gasket 155, a crinkling occurs in the gasket 155 at a portion where the projection of the current collector 124 and the concave of the gasket 155 are engaged with each other. The crinkling makes a gap occur between the current collector and the gasket, to thereby make it easy to separate the current collector and the gasket from each other.
Furthermore, because the capacitor of the conventional example 4 is structured such that the outer diameters of the current collector 124 and the gasket 155 are identical with each other as in the structure of the capacitor in the conventional example 1, and the gasket 155 and the current collector 124 which are different in material from each other are bonded together, a separation is liable to occur between the gasket 155 and the current collector 124 due to the thermal stress or the like.
The present invention has been made under the above circumstances, and therefore an object of the present invention is to provide an electrical double layer capacitor, which is capable of improving the productivity.
Another object of the present invention is to provide an electrical double layer capacitor that improves the reliability even under a high temperature environment.
To achieve the above objects, according to the present invention, there is provided an electrical double layer capacitor as follows.
A cell member of a couple of polarized electrodes sandwiching a porous separator is accommodated within a first gasket of a frame shape and current collectors, a first current collector and a second current collector are adhered on an upper end side and a lower end side of this gasket, respectively.
In addition, additional frame gaskets, a second gasket and a third gasket are adhered on the upper end side and the lower end side of the first gasket to sandwich the first current collector and the second current collector with the first gasket, respectively, sealing the inside of the first gasket.
And the electric double layer capacitor of the present invention has a structure that a step portion is formed in an inner peripheral end surface of at least one of the gaskets confronting and the side edge of each collector is accommodated by this step portion. The positioning of the current collector in the gasket may become easy and bonding of the current collector to the gasket as a low stress by this step portion achieved.
In addition, the electric double layer capacitor of the present invention has a structure that all surfaces of the side edge of each current collector are contacted with the gaskets confronting, which improves the reliability of the electric double layer capacitor by preventing the leaking of the electrolytic solution from the capacitor cell under the high temperature environment.
Especially, an electric double layer capacitor of high reliability under the high temperature environment may be obtained by adjusting the ratio to the thickness of the current collector of the depth of the step portion to 0.1xcx9c3.0 in the electric double layer capacitor of the present invention.